Energy storage systems, including electrochemical devices such as Lithium-ion batteries having high energy densities, increasingly have to comply with safety requirements to meet the growing demand for large-size electrochemical cells.
One of the most critically important components to ensure safety of an electrochemical cell is the separator, whose primary function is to prevent physical and electric contact between the positive electrode and the negative electrode of the electrochemical cell while permitting electrolyte ions to flow there through.
The separator must be chemically and electrochemically stable towards the electrolyte and the electrode materials and must be mechanically strong to withstand high tensions generated during battery assembly operations. Also, its structure and properties considerably affect battery performances, including energy density, power density, cycle life as well as safety.
For high energy and power densities, the separator is required to be very thin and highly porous while still remaining mechanically strong.
For battery safety, the separator should be able to shut the battery down when overheating occurs so that thermal runaway, causing dimensional shrinking or melting of the separator, which results in physical contact of the electrodes, and the resulting internal short circuit can be avoided.
Also, a low thickness of the separator is required for high energy and power densities. However, this adversely affects the mechanical strength of the separator and the safety of the battery thereby provided.
Inorganic composite membranes have been widely used as separators for electrochemical devices including secondary batteries, in particular Lithium-ion batteries.
A variety of inorganic filler materials have been long used to fabricate inorganic composite membranes wherein inorganic particles are distributed throughout a polymeric binder matrix.
Although inorganic composite membranes offer excellent wettability by the electrolytes, good thermal stability and zero-dimensional shrinkage at high temperatures, they are usually not mechanically strong enough to withstand handling in cell winding and assembly.
In many cases, the inorganic composite membrane contains a very high content of inorganic filler materials. In some instances, the inorganic composite membrane so obtained exhibits poor mechanical strength.
One particular challenge has been to provide for multi-layer composite membranes with acceptable thickness to be suitably used as separators in electrochemical devices.
Multilayer composite membranes can be obtained using multiple coating steps. However, multiple steps disadvantageously increase processing costs.
For instance, US 2013/0023620 (SOLVAY SPECIALTY POLYMERS ITALY S.P.A.) 24 Jan. 2013 discloses use of fluoropolymer hybrid organic/inorganic composites for the manufacture of separators for Lithium-ion batteries, said fluoropolymer hybrid organic/inorganic composites being obtainable by reacting functional fluoropolymers having hydroxyl groups with hydrolysable compounds of Si, Ti or Zr.
Also, WO 2013/072216 (SOLVAY SPECIALTY POLYMERS ITALY S.P.A.) 23 May 2013 discloses polymer electrolyte separators based on fluoropolymer hybrid organic/inorganic composites obtainable by reacting functional fluoropolymers having hydroxyl groups with hydrolysable compounds of Si, Ti or Zr.
Further, US 2012/0003524 (KOREA INSTITUTE OF SCIENCE AND TECHNOLOGY) 5 Jan. 2012 discloses a process for manufacturing by electrospinning ultrafine fibrous composite separators from a mixture of a metal oxide and a polymer resin solution.
Similarly, MONTICELLI, O., et al. Preparation, characterization and properties of nanofibers based on poly(vinylidene fluoride) and polyhedral oligomeric silsesquioxane. Polymers for Advanced Technologies. Dec. 8, 2011, vol. 23, no. 9, p. 1252-1257. discloses electrospun composite nanofibers based on poly(vinylidene fluoride) and polyhedral oligomeric silsesquioxanes (POSS) such as epoxycyclohexylisobutyl POSS.
There is thus still the need in the art for an alternative process for manufacturing membranes having high porosity and thus high ionic conductivity to be suitably used as separators in electrochemical devices while maintaining outstanding thermo-mechanical properties during operation of the same.